Ingore jemalloc make distclean error in Makefile, caused by make clean against a tree that never compiled jemalloc.

Author: antirez

Makefile

@@ -12,7 +12,7 @@ clean:
 	cd src && $(MAKE) $@
 	cd deps/hiredis && $(MAKE) $@
 	cd deps/linenoise && $(MAKE) $@
-	cd deps/jemalloc && $(MAKE) distclean
+	-(cd deps/jemalloc && $(MAKE) distclean)
 
 $(TARGETS):
 	cd src && $(MAKE) $@